The present disclosure relates to systems and methods for magnetic resonance imaging (MRI). More particularly, the present disclosure relates to systems and methods for improved parallel imaging, particularly, in the presence of an inhomogeneous magnetic field.
Magnetic resonance imaging (MRI) of non-ferrous metallic implants is challenging because of the substantial inhomogeneity induced in the B0 magnetic field of the MRI system. This inhomogeneity makes signal excitation difficult and leads to severe off-resonance in nearby tissue and distorts conventional spatial-encoding mechanisms. The image artifacts resulting from the off-resonance can significantly degrade the diagnostic quality of an image, making clinical diagnoses in the presence of metal very challenging.
Methods such as view angle tilting (VAT), slice-encoding for metal artifact correction (SEMAC), and multi-acquisition variable-resonance image combination (MAVRIC) have been developed in an attempt to mitigate the off-resonance artifacts surrounding metallic implants. These methods are described, for example, by K. M. Koch, et al., in “Magnetic Resonance Imaging Near Metal Implants,” J Magn Reson Imaging, 2010; 32(4):773-787.
Due to safety and hardware limitations, a single radio frequency (RF) pulse is often incapable of exciting the wide range of frequencies near metal. To cover this broad spectrum of frequencies, methods such as MAVRIC utilize multiple acquisitions, each with an RF pulse at different resonance frequency offsets. Therefore each acquisition produces images with a unique spectral sensitivity pattern. These “spectral” images can then be combined to generate a composite image with the combined signal images acquired at the different frequencies. Although MAVRIC is capable of mitigating artifacts caused by large perturbations in the B0 field, it requires long scan times and is, thereby, limited in spatial resolution.
In addition to lengthy scan times, MAVRIC also utilizes a frequency-encoding (readout) gradients, which fundamentally limits its ability to eliminate in-plane signal loss and pile-up. While VAT and Jacobian methods help reduce in-plane signal loss and pile-up errors near metal, signal loss is unavoidable when the local B0 gradient within a voxel exceeds the readout gradient, such as in tissue directly adjacent to a metal object, such as described in Koch, K. M., King, K. F., Carl, M. and Hargreaves, B. A. (2014), Imaging near metal: The impact of extreme static local field gradients on frequency encoding processes. Magn Reson Med, 71: 2024-2034. doi: 10.1002/mrm.24862, which is incorporated herein by reference in its entirety.
Single point imaging (SPI) techniques encode k-space one point at a time by eliminating frequency-encoding gradients and have been previously proposed in an effort to produce distortion-free images in the presence of off-resonance. This effort, however, has not gained traction because of prohibitively long scan times associated with SPI methods. A recent SPI method that is capable of spectrally-resolved, purely phase-encoded three-dimensional acquisitions was recently proposed, as described in co-pending U.S. patent application Ser. No. 13/451,773, filed on Apr. 20, 2012, entitled “System and Method for Spectrally-Resolved Three-Dimensional Magnetic Resonance Imaging Without Frequency-Encoding Gradients,” and which is herein incorporated by reference in its entirety. Similar to MAVRIC, this new spectrally-resolved purely phase-encoded technique can utilize multiple acquisitions each with an RF pulse at different center frequency offsets, to cover a broad spectrum of frequencies new metal. However acquiring multiple acquisitions can increase scan time even further.
Thus, there remains a need for a system and method for magnetic resonance imaging that is capable of accelerating data acquisitions in the presence of severe off-resonances, such as those caused by magnetic field inhomogeneities induced by a metallic object.